A line has slope 2. A second line makes an angle θ with it, where tanθ = 1/3…

The slope of the line through $(x_1, y_1)$ and $(x_2, y_2)$ is $m = \dfrac{y_2 - y_1}{x_2 - x_1}$ when $x_1 \neq x_2$. It equals $\tan\theta$, where $\theta$ is the angle of inclination. The acute angle between lines of slopes $m_1$ and $m_2$ has $\tan\theta = \left|\dfrac{m_2 - m_1}{1 + m_1 m_2}\right|$.